Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle-wheel sets per vehicle with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (e.g., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way.
Maximum tractive or braking effort is obtained if each powered wheel of the vehicle is rotating at such an angular velocity that its actual peripheral speed is slightly higher (motoring) than the true vehicle speed (i.e., the linear speed at which the vehicle is traveling, usually referred to as “ground speed” or “track speed”). The difference between tractive wheel speed and track speed is referred to as “creepage” or “creep speed.” There is a variable value of creepage at which peak tractive effort is realized. This value, commonly known as the optimal creep setpoint is a variable that depends on track speed and rail conditions. So long as the allowable creepage is not exceeded, this controlled wheel slip is normal and the vehicle will operate in a stable microslip or creeping mode. If wheel-to-rail adhesion tends to be reduced or lost, some or all of the tractive wheels may slip excessively, i.e., the actual creep speed may be greater than the maximum creep speed. Such a gross wheel slip condition, which is characterized in the motoring mode by one or more spinning axle-wheel sets, can cause accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive effort.
The peak tractive effort limits the pulling/braking capability of the locomotive. This peak tractive effort is a function of various parameters, such as weight of the locomotive per axle, wheel rail material and geometry, and contaminants like snow, water, grease, insects and rust. Contaminants in the wheel/rail interface reduce the maximum adhesion available, even at the optimal creep setpoint.
Locomotives used for heavy haul applications typically must produce high tractive efforts. Good adhesion between each wheel and the surface of a railroad rail contributes to the efficient operation of the locomotive. The ability to produce high tractive efforts depends on the available or potential adhesion between the wheel and rail. Many rail conditions such as being wet or covered with snow or ice require an application of friction enhancing agent such as sand to improve or enhance the adhesion of the wheel to the rail. Therefore, locomotives typically have sand boxes on either end of the locomotives, and nozzles to dispense the sand to the rail on either side of a locomotive truck.
Locomotives may enhance the adhesion between their wheels and the railroad rail by initiating a flow of sand from the sand boxes to the rail surface. The flow of sand may be initiated in response to one or more conditions being met such as one or more wheel axels slipping. When such condition is met, typical sanding systems will activate a flow of sand through two sand applicators located in front of each of two locomotive trucks when the locomotive is moving forward. Sand is thus dispensed at a fixed rate through four sand applicators each time there is a demand for sanding from the locomotive controller. Sand is typically dispensed for a set period of time, which frequently results in more sand being dispensed than necessary to maximize adhesion between the locomotive wheels and the railroad rail.
Dispensing more sand than is necessary is wasteful and may cause sand to be delivered to areas that are undesirable. For example, typical systems that automatically or manually dispense sand in response to a condition being met may cause sand to get into switches, track circuits or drains, for example, which may damage equipment or lead to malfunctions.